du -h --max-depth=1 -x /
du -h --max-depth=1 -x /home
Did you find what folder consumes the disk space the most?
Code:du -h --max-depth=1 -x /
You will get a list of folders. Go deeper into a directory which has the biggest size, e.g. /home
Code:du -h --max-depth=1 -x /home
and so on. When you find what directory/files use the most of disk space then post a list of them here.
If any of users is abusing the server then you should investigate it. Probably their account/site is compromised. There is no way to guess real reasons of the issue without logging into your server.
As a quick solution change PHP settings of the user to make session files to write in their homedirs: /home/username/tmp/
As for the debugging PHP scripts kindly check this:
- PHP: Debugging in PHP - Manual
http://php.net/manual/en/debugger.php
p.s. Since it's a WordPress causing the issue the thread is now moved to General Technical Discussion & Troubleshooting subforums.